Electrostatic transducer and method and means for making same

ABSTRACT

An electrostatic loudspeaker unit of the push-pull type employing grids of spaced insulated wires disposed on each side of a flexible diaphragm. The electrode grids are in the form of a flat spiral so as readily to accommodate effects of variations in temperature without causing the wires to move out of their designed plane. The electrode grids are preformed using a special tool, a part of which is utilized to mount the spiral electrode in the support frame with a high degree of precision.

United States Patent [191 J anszen [451 Mar. 26, 1974 ELECTROSTATICTRANSDUCER AND METHOD AND MEANS FOR MAKING SAME [75] Inventor: Arthur A.Janszen, Belmont, Mass.

[73] Assignee: Electrostatic Research Corporation,

Salem, Mass.

22 Filed: June 28,1971

21 Appl.No.: 157,191

[52] US. Cl 179/111 R [51] Int. Cl H04r 19/00 [58] Field of Search179/111 R [56] References Cited UNITED STATES PATENTS 1,708,846 4/1929Nagel 179/111 R 2,000,437 5/1935 Colvin 179/111 R FOREIGN PATENTS ORAPPLICATIONS 611,249 3/1935 Germany 179/111 R 808,593 7/1951 Germany179/111 R Primary Examiner-Ralph D. Blakeslee Attorney, Agent, orFirmKenway, Jenney & Hildreth 5 7 ABSTRACT An electrostatic loudspeakerunit of the push-pull type employing grids of spaced insulated wiresdisposed on each side of a flexible diaphragm. The electrode grids arein the form of a fiat spiral so as readily to accommodate effects ofvariations in temperature without causing the wires to move out of theirdesigned plane. The electrode grids are preformed using a special tool,a part of which is utilized to mount the spiral electrode in the supportframe with a high degree of preci- 7 Claims, 8 Drawing Figures mimwmzsm3800402 SHEET 1 or 3,

INVENTOR ARTHUR A. JANSZEN ATTORNEYS ELECTROSTATIC TRANSDUCER AND METHODAND MEANS FOR MAKING SAME BACKGROUND OF THE INVENTION In the presentinventors prior US. Pat. Nos. 2,631,196 and 2,896,025, there aredisclosed electrostatic loudspeakers characterized by fixed electrodesin the form of grids of insulated wires disposed on opposed sides of aflexible diaphragm to provide a pushpull transducer. The second patentis especially concerned with a construction that permits the use ofclosely-spaced wires arranged in straight, parallel relation whileavoiding the effects of thermal expansion and contraction that wouldnormally cause the wires to move out of their desiredprecisely-determined flat plane. In particular, the method and structuretaught by the patent requires winding carefully pre-straightenedinsulated wire onto a pair of support frames having a multiplicity ofaccurately formed notches to receive the wires. Subsequent steps involvecutting the insulating sheath on each wire, soldering the exposedportion of each wire to a cross-bar, cementing the insulating sheath ofeach wire to the frame at each notch region, and finally cutting thewires along lines inwardly of the frame edge. The wires, being anchoredat the bus bar end only, are free to slide endwise within their sheathsand thus prevent buckling that would otherwise result from differentialchanges in dimensions of wire and support with variations intemperature.

It is evident that this procedure, when carefully carried out, insuresthat the wires are not caused by thermal expansion to distort or bendout of the precise plane so essential to proper operation of theelectrostatic transducer. It is difficult, however, to manufacture thesupport frames so that the slots are of the proper depth to provide thedesired spacing of the wires from the diaphragm when the frames areassembled, and exceptional care is needed in stripping and soldering themany wires that make up the grids, as well as in properly securing thesheaths at the numerous crossing points of the supports.

SUMMARY OF THE INVENTION The present invention is concerned withimprovements in the construction and configuration of electrostaticloudspeakers, and especially the improvements and advantages that resultfrom a novel electrode configuration that avoids the difficultiesassociated with the use of electrode grids employing straight parallelwires to form a rectilinear electrode configuration.

A principal feature of the invention is the provision of flat spiralwire grid electrodes, each portion of which is a segment of an are,formed by winding a continuous insulated wire into a flat spiral. Theprovision of flat spiral electrodes permits the diaphragm to be clampedboth at its center and ,at a circular outer margin, as well as at anintermediate concentric ring if desired, so that the diaphragm may flexin one or more annular bands or regions. The use of electrodes formedout of wire wound in a fiat spiral eliminates the problems arising outof differential thermal expansion and contraction, as the curvature ofthe wires between supports permits any relative motion between wire andframe motion due to thermal effects to take place in the plane of thespiral, without altering the critical spacing between electrode anddiaphragm.

Another feature of the invention resides in a novel technique, andspecial tool for carrying out the same, by which the flat spiral gridelectrodes are accurately formed and mounted on the support frames in amanner which insures that the grids are secured in extremely precisespatial relation to the diaphragm plane. As a further feature, thesecuring of the insulated wires to the frames at each of the supportpoints is readily carried out during the mounting procedure.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a support frame foruse in an electrostatic transducer of the type in which the diaphragmand the electrodes are arranged in two annular bands or regions, showingportions of the spiral electrode mounted on the support.

FIG. 2 is an enlarged sectional view of an assembled unit, taken fromthe center outwardly on a line corresponding to the line 22 of FIG. 1,showing the positioning of the spiral electrodes within their supportframes.

FIG. 3 is a sectional detail view of a portion of an alternative form ofelectrostatic transducer embodying the invention, wherein the diaphragmis clamped only at its center and at a circular outer rim region.

FIG. 4 is an exploded view of one form of special winding tool, suitableforming spiral electrodes having two annular bands.

FIG. 5 is a view of the tool on a somewhat larger scale than FIG. 4,showing in greater detail the faces of the inner and outer circularmembers of the winding tool, (the two elements shown separated at theright in FIG. 4) with portions of the windings shown in the starting,cross-over and finishing regions.

FIG. 6 is an enlarged sectional view of the winding tool with the partsin operative relation for forming the inner portion of a spiralelectrode having spaced annular regions.

FIG. 7 is a corresponding view with the tool components in position forcarrying out the forming of the outer region of the spiral electrode.

F IG. 8 is a view of another form of electrostatic transducer employingspiral electrodes, wherein the electrode support region consists of anapertured or perforated structure.

DESCRIPTION OF THE INVENTION 1. The Transducer The electrostatictransducers of the present invention resemble in some respects those ofmy prior US. Pat. No. 2,896,025. A very thin flexible diaphragm having aconductive coating of high resistivity serves as the sound-radiatingelement as well as one electrode. The diaphragm is mounted betweensupport frames which carry the acoustically-transparent wire grids thatserve as the fixed electrodes, energized in a conventional manner byelectrical signals from a push-pull source. A high voltage biaspotential applied between the diaphragm and the grid electrodesmaintains the requisite high voltage across the air gaps betweendiaphragm and fixed electrodes, as disclosed in said patent.

According to the present invention, the support frames illustrated inFIGS. 1 and 2 are distinguished by their provision of circular clampingmeans for the diaphragm 12. A central disk region 14 and a coplanarouter ring 16 on each frame provide the circular center and outerclamping surfaces between which the diaphragm is secured. Outwardly ofthe outer clamp ring 16, the margins 22 of the frames may be square or,if desired, of other configuration, depending on mounting requirementsor other considerations.

The support frames, which may advantageously be molded fromdimensionally stable insulating material, are formed with a plurality ofradially disposed integral ribs 24 on which the spiral electrodes aremounted. For large-size low frequency radiators, and for units such ashigh frequency radiators where it is desired to divide the radiatingsurface of the diaphragm into a plurality of annular bands, the framespreferably also include a ring 26 intermediate the central disk 14 andthe outer clamping ring 16. This permits a greater number of ribs to bedisposed between the intermediate ring 26 and the outer ring 16 thanwithin the annular region inwardly of the intermediate ring, in orderthat the arcuate distances between the support ribs may be more nearlythe same over the entire electrode region.

For low frequency radiators the intermediate ring 26 will not projectabove the surfaces of the ribs 24 in order that the diaphragm may flexin a single annular band, as illustrated in FIG. 3. For tweeter units,where the diaphragm is advantageously divided into two annular radiatingbands, the surface of the rib 26 extends into the plane of the centralsupport 14 and outer rim 16 so as to provide an intermediate circularclamping region for the diaphragm, as illustrated in FIG. 2. In suchevent, it is desirable at the intersections between radial ribs 24 andintermediate rim 26 to provide holes 34 adapted to receive staking pins36 which secure the frames together in the intermediate ring region whenthe frames are subsequently assembled with the diaphragm clampedbetween.

The fixed electrodes, like those of my prior patent, are preferablyformed of single strand copper wire having a dielectric insulatingsheath of vinyl chloride composition. A wire size of No. 28 A.W.G. isgenerally satisfactory for tweeters and head phone units; a somewhatheavier gauge is desirable for the larger low frequency radiators. Thewire is wound into a flat spiral of uniformly spaced turns so as tooccupy the open annular region or regions between the center and theouter rim of each support frame. By the provision of these flat spiralwire grid electrodes, it becomes practical for the first time, it isbelieved, to employ what are in effect circular wire grid electrodes, inconjunction with circular flexing regions of the radiating diaphragm.Not only are such flat spiral electrodes free of any tendency for thewires to distort out of the flat plane by reason of differential thermalexpansion, but also such configuration permits the electrodes to beformed out of a single continuous wire that eliminates the necessity formultiple soldered connections to a common bus.

A further feature, which not only simplifies the construction but alsois responsible for the high degree of precision with which the wire gridelectrodes are mounted in a plane parallel to and accurately spaced fromthe diaphragm, results from the fact that the radial support ribs 24 arenot slotted or notched to receive the insulated sheaths of the wires. Inthe construction shown in my prior US. Pat. No. 2,896,025, the depth ofthe slots determines the ultimate spacing of the wires from thediaphragm, and the requisite accuracy is difficult to provide.

In my present invention, not only are the ribs 24 not required to beslotted, but the surfaces or edges of the ribs adjacent the wires arenot required to engage the insulating sheaths on the wires to determinetheir supported position. The frames are preferably so designed that, asbest illustrated in FIGS. 2 and 3, the perpendicular distance betweenthe plane of the rib edges and the plane of the diaphragm supportsurfaces 14 and 16 is slightly greater than the distance from thediaphragm to a plane tangent to the insulating sheaths on the side awayfrom the diaphragm, for the desired spacing of the conductive wires fromthe diaphragm in the completed transducer. The desired relativelycritical spacing between electrode wires and the diaphragm iseffectively attained by the procedures to be described wherein the wiresare positioned with their insulating sheaths in spaced relation to theribs 24 through the use of an intervening hardenable material 40 withinwhich the spiral electrode grids are embedded solely with reference totheir distance from the plane of .the diaphragm support surfaces 14 and16 and without regard to the resulting spacing or variations in spacingof the insulated wires from the adjacent edges of the ribs of thesupport frame.

2. The Electrode-Forming Tool The electrode grids are fabricated withina special winding tool having spaced walls extending outwardly from acentral hub to define a narrow slot or guideway in which the insulatedwire can be wound in a manner to produce the desired flat spiral havinguniform spacing between turns. The winding tool to be described isadapted for making the dual spiral windings of the type shown in FIGS. 1and 2, in which the diaphragm is supported by an intermediate rim sothat the electrode grid is disposed in inner and outer annular bands. Bya simple modification, a tool for forming a continuous, single spiralwinding may readily be provided.

The winding tool shown in FIGS. 4-7 employs circular members havingcooperating coaxial connecting means by which the members may beassembled to form an accurately-defined narrow slot within which thespiral winding can take place. Upon completion of the winding operation,the tool can be disassembled and part of the tool, to which the woundelectrode has been caused to adhere, then utilized to transfer thespiral electrode to the support frame and to position it within theframe in precise relationship to the plane of the clamping surfacesbetween which the diaphragm is supported in the completed transducer.

In the exploded view of FIG. 4, the circular member at the left,indicated generally at 52, provides one wall of the guideway and servesalso as the transfer means, while members 54 and 56, shown at the right,cooperate to define the other wall of the winding slot with the innermember 54 being operative during the formation of the winding for theinner annular region, while the member 56 having the outer annular faceis utilized during the winding of the outer annular portion of thespiral electrode.

As best shown in FIGS. 6 and 7, the guideway face of tool member 52 hascoplanar inner and outer annular surfaces 62 and 64, separated by acircular channel 66 whose dimensions and radial position correspond tothose of the intermediate annular ring 26 of the support frame. Inspaced opposing relation to the inner annular face 62 of member 52 isthe guideway surface 72 of member 54, while the outer annular face 74 ofmember 56 opposes surface 64 of member 52. The annular surface 74terminates at its inner edge in a projecting annular shoulder 76 whichregisters with the channel 66 in member 52. The inner member 54 islocated inwardly of rim 76 within a circular recess 82 formed in member56. The inner member 54 is provided with a short projecting hub 84adapted to fit within a coaxial recess 86 of corresponding diameter atthe center of member 52 to insure precise coaxial registration ofmembers 52 and 54. Bolt 88 extends through member 52 into a threadedrecess in the center of member 54 to secure the parts during the windingoperation.

The outer member 56 is mounted on the inner member 54 in a manner whichpermits precise axial adjustment, for a purpose to be described. The hub92 of the member 56 is bored to receive the shaft 94 of the innerflanged member 54. A bolt 96 extends through the outer end of hub 92into threaded engagement with the end of shaft 94, while a compressionspring 98 urges the outer member 56 in an axial direction away frominner member 54 to establish a relative position determined by theadjustment of bolt 96. Thus rotation of the bolt by a suitable wrenchpermits accurate axial adjustments to be made to position the outermember 56 in first one and then the other of the positions shown in theenlarged views of FIGS. 6 and 7. One or more pins 102 extend intoregistering apertures 104 in the central regions of members 52, 54 and56 to insure that the parts can be assembled in only a singlepredetermined angular relationship about the common center.

3. Winding the Spiral Electrode In preparation for the operation ofwinding the spiral electrode within the special tool it is necessary,before the tool is assembled, to place a suitable coating on thesurfaces 62 and 64 of the member 52, to which the completed winding maybe caused to adhere in order to facilitate the transfer of the woundspiral to its proper position on the support frame. A thin coating of awax having a melting point within the range of 100C to 200C has beenfound suitable. The free end of the insulated wire 112 is then insertedfrom the coated face of the disk out through a passage extending throughmember 52 approximately one sheath diameter outwardly of the edge of thecircular recess 86 that receives the end 84 of member 54. The end of thewire is then secured to a suitable anchor point such as indicated at 114in FIG. 3.

In addition, a spacer strand 118 of round plastic material of a diametersubstantially that of the sheath is anchored in the member 54 by runningthe free end through a passage at the base of the guideway face 72,continuing through an aligned passage in member 56, and forming a knot120. The strand can then be pulled back until the knot seats against theoutside wall of member 56.

The disk 52 is now assembled with members 54 and 56, using clamping bolt88 to draw the members 52 and 54 into firm engagement at the hub region.The adjusting bolt 96 at the right hand end of the assembly is nowturned until members 54 and 56 are in the relation shown in FIG. 6 withthe surface 72 approximately flush with the side of rim 76. In thisposition, the rim does not obstruct access to the winding passagedefined by the opposing faces 62 and 72 of disk 52 and member 54respectively.

It is important to note that the spacing between these surfaces 62 and72 is greater than the diameter of the insulating sheath on the wire.This is to permit the winding to be carried out in a manner whichresults in the turns of the wire being disposed toward the surface 62 ofmember 52 while the turns of the spacing material are offset in theother direction so as to rest against the opposite surface 72 of member54. This oblique or staggered relation of wires and spacer strand notonly provides the desired spacing between turns of the flat spiral, butalso prevents jamming of the spacer strand between turns of theconductor, which would interfere with the subsequent separation of thespacer strand from the wound spiral, upon disassembly of the tool.

By way of example, with a conductor diameter (outside diameter ofsheath) of approximately 0.023 inch and a similar diameter of the spacerstrand, a slot width or spacing between guideway surfaces of the orderof 0.038 inch is desirable. The resulting space between turns of thespiral is approximately 0.015 inch.

It has been found that the conductor and the spacing strand may be woundin the desired offset relation by leading the wire, which has beensuitably straightened, close to the left hand side of the slot, asviewed in FIG. 6, while the spacer strand is fed in close to the righthand edge. To aid in the proper formation of the turns at the beginningof the winding, the region of the projecting hub of member 54 inwardlyof the circular end 84 is formed with a spiral ramp 124 ending close tothe starting point of the winding, as shown in FIGS. 4 and 5. Thiscauses the paired conductor and spacer strand during the first rotationof the tool to rise gradually to provide a smooth transition as thesecond turn is laid over the starting turns of conductor and spacer.Note also the previously-described radial offset in the positions oflead-in holes for conductor and spacer, shown in enlarged detail in FIG.5, which establishes the initial relationship of wire and spacingstrand.

To carry out the winding, the tool is mounted by its hub 92 in apower-driven rotary support by which the tool may be rotated to causethe winding to proceed until the space between surfaces 62 and 72 hasbeen substantially filled. The driving means is now stopped and the toolslowly rotated until the conductor and the spacer strand, still in theirsuperposed relation, are seen to be aligned with a cross-over slot 126extending obliquely through the projecting rim 76.

Once the cross-over slot has been brought into alignment with theconductor and spacer strand, the adjusting bolt 96 is rotated to movemember 56 toward member 52 until the rim 76 seats in the recess 66 andthereby establish the desired winding space between outer surfaces 64and 74 on member 52 and 56 respectively. The winding of the outer regionof the grid may now proceed in the same manner as with the forming ofthe inner annular region. Advantageously the radially outer surface ofrib 76 in the region approaching the exit end of the cross-over slot 126may have a rising ramp region 130 (see FIG. '4) to facilitate a smoothtransition at the commencement of the second turn, just as results fromthe ramp 124 for the first turn of the inner winding.

When the required number of turns have been wound to fill the windingspace outwardly of rim 76 to the margin of surface 64, the tool rotationis discontinued and the conductor brought out through a peripheraloblique slot 132 in member 52 and suitably anchored to retain theconductor under adequate tension, after which the conductor may be outfree beyond the anchor point.

4. The Transfer Procedure To carry out the mounting of the spiralelectrode on the support frame, it is necessary that the wound electrodebe retained on the surface of member 52 when the tool is disassembled.If the thin wax coating applied to the surface of the tool member 52preparatory to the winding is relatively soft and sticky, the wires willhave been caused to adhere to the surface of member 52 during thecarrying out of the winding operation. If the wax coating is of asomewhat harder composition, the wax may be softened by passing acurrent through the wire and thereby cause the wax to bond where it isin contact with the insulating sheath. The securing bolt 88 is thenloosened and removed, which permits transfer member 52 to be separatedfrom the parts 54 and 56 of the winding tool. Due to the staggeredrelation of conductor and spacer strand, the strand 118 freely releasesfrom between the turns of the conductor spiral, leaving the spiralelectrode adhering to the face of member 52.

A support frame is prepared by applying a hardenable material 40, suchas an epoxy cement, to the top surfaces of the radial ribs 24 of thesupport frame. The member 52, now serving as a carrier, is thenassembled with the frame so that the spiral grids are in opposed facingrelation to the cement-coated support ribs. The frame may be provided inits central region 14 with apertures 134 which register with the guidepins 102 in member 52 to aid in aligning the frame and transfer memberin proper relation during assembly so that the grids are properlycentered with the cross-over region of the winding located to enter thecross-over slot 136 provided for the purpose in the intermediate rim 26and the outer end of the winding passing out through slot 138 in theoutside rim 16.

The frame and the member 52 are then clamped together with the centralregion 14 of the frame seated in the central recess 86 in the member 52,the intermediate rim 26 seated in the channel 66, and the outer rim l6seated on the marginal surface 140 of the member 52. By this engagementof circular surfaces on tool and electrode support, the electrode gridsare caused to become embedded in the still-soft cement 40 the precisedistance that provides the desired design spacing of the electrode gridsfrom the surface of the diaphragm. As the support ribs have preferablybeen fabricated with their edge surfaces disposed slightly further fromthe plane of the diaphragm support surfaces than would provide thedesired position of the conductors if the insulating sheaths resteddirectly on the ribs, the hardenable cement bond, utilized inconjunction with the positioning of the transfer tool solely byreference to the surfaces on which the diaphragm is supported, providesthe accomodation which eliminates the need for close tolerances in ribedge position and insures positive and precise mounting of the grids ontheir support frames. The assembly is then allowed to set for sufficienttime to permit the cement to harden. Thereafter, by a heating currentthrough the electrode, the wax may be softened to release the wires fromthe transfer tool and permit removal of the frame from the tool 52.

The final steps of applying a tensioned diaphragm, suitably coated witha slightly conductive coating, to one of the frames, and applying theother clamping frame are all taught in my prior US. Pat. No. 2,896,025.The support frames are provided with holes 142 located in the marginalregion of the frame outwardly of the outer rim 16 to receive terminals144 for connection to the outer end of the wire electrode and to theconductive coating of the diaphragm substantially as shown in thatpatent.

As has previously been indicated, electrostatic transducers formed inaccordance with my invention to serve as low frequency radiators willgenerally employ a diaphragm that is supported only at its center and atan outer circular rim, without intermediate support. Thus theintermediate rib 26, desirable for providing intermediate support forthe radial ribs 24, will not project above the surface of ribs 24, butwill be as shown in FIG. 3, with the spiral electrode having acontinuous uniform spacing of turns from center frame support 14 toouter rim 16. In this sectional detail, it will be seen that the cement40 is disposed both on the radial ribs 24 and on the intermediate rib 26so as to provide bonding support to all turns of the spiral windingwhenever the wires are in proximity to a rib edge.

For winding the electrodes for low frequency radiators, and also forsmall-diameter radiators suitable for electrostatic headphones, thewinding tool may not need provision for successive adjustments of thewinding slot spacing, such as have been described and shown'where thewinding has to accomodate an intermediate support rib for the diaphragm.For a single continuous spiral, the tool member 52 will omit recess 66,leaving surfaces 62 and 64 to form a single continuous surface. Members54 and 56 can be combined in a single tool having no projecting rim 126and with surface 74 coplanar and continuous with surface 72.

For small-size transducers, for example units especially suitable foruse in electrostatic headphones, the modified form of support frameillustrated in FIG. 8 may be employed. In this construction, instead ofa plurality of radial ribs where edges are coated with a hardenablecement or adhesive to receive the wires, as shown in FIGS. 1, 2 and 3,the region 148 of the frame intermediate the center 150 and the margin152 may simply be a perforate or apertured area, across which narrowbands 154 of cement are laid radially to receive and support thespirally-wound electrode. The openings 156 which provide the acoustictransparency for the support may be readily formed during the molding ofthe frame, and can be circular or of other shape, as well as of uniformor of varying size.

It will be apparent from the foregoing that a new form of electrostatictransducer has been provided wherein the fixed electrodes comprise gridsof spaced insulated wires disposed in curvilinear rather thanrectilinear arrangement, thereby insuring that differential thermalexpansion does not force the wires out of their design plane. Byachieving the curvilinear arrangement of wires through forming theelectrode in a flat spiral of spaced turns disposed in an annular band,the diaphragm may advantageously be supported at its center and at acircular rim close to the outer boundary of the electrode.

The invention likewise comprises, as a further feature, a novel tool andtechnique for conveniently and precisely forming the spiral electrodeand for accurately mounting it in its support frame, so as to greatlysimplify the manufacturing procedures for electrostatic transducers ofvarious sizes and types while insuring that the requisite toleranceswill be consistently attained.

I claim:

1. An electrostatic transducer comprising a flexible diaphragm having aconductive coating of high resistivity, a pair of frames each having acircular rim between which the margin of the diaphragm is clamped and acircular center region coplanar with the rim between which the center ofthe diaphragm is clamped, and conductive electrodes mounted on theframes on each side of the diaphragm intermediate the rim and the centerregion in closely-spaced planar relation to the diaphragm, the frameshaving electrode support means intermediate said rim and center region,each electrode comprising a continuous wire having an insulating sheath,disposed in a flat spiral having spaced turns, the electrode supportmeans being spaced from the diaphragm a greater distance than thedistance from the diaphragm to a plane tangent to the insulating sheathsof the wires on their sides remote from the diaphragm, and bondingmaterial intermediate the sheaths and the support means.

2. An electrostatic transducer according to claim 1 wherein the bondingmaterial is disposed in radial strips between center region and the rim.

3. An electrostatic transducer according to claim 1 wherein the supportmeans for the electrodes comprise spaced radially disposed membersintermediate the central and rim portions of the frames and the bondingmaterial is intermediate the radially disposed members and adjacentportions of the insulating sheaths.

4. An electrostatic transducer according to claim 1 wherein theelectrode support means comprises a web having a multiplicity of smallclosely-spaced apertures and the bonding material is disposed in radialstrips on the web.

5. An electrostatic transducer according to claim 1 wherein each framehas an intermediate ring portion between the central region and theouter circular rim.

6. An electrostatic transducer according to claim 1 wherein a pluralityof radial supports extend between central disc and intermediate ring,and between the intermediate ring and the outer rim and the bondingmaterial is intermediate the radial supports and adjacent portions ofthe insulating sheaths.

7. An electrostatic transducer comprising a flexible diaphragm having aconductive coating of high resistivity, a pair of frames each having acircular rim between which the margin of the diaphragm is clamped and acircular center region coplanar with the rim between which the center ofthe diaphragm is clamped, and conductive electrodes mounted on theframes on each side of the diaphragm intermediate the rim and the centerregion in closely-spaced planar relation to the diaphragm, the frameshaving electrode support means intermediate said rim and center region,each electrode comprising a continuous wire disposed in a flat spiralhaving spaced turns, the electrode support means being spaced from thediaphragm a greater distance than the distance from the diaphragm to aplane tangent to the insulating sheaths of the wires on their sidesremote from the diaphragm, and bonding material intermediate the sheathsand the support means.

1. An electrostatic transducer comprising a flexible diaphragm having aconductive coating of high resistivity, a pair of frames each having acircular rim between which the margin of the diaphragm is clamped and acircular center region coplanar with the rim between which the center ofthe diaphragm is clamped, and conductive electrodes mounted on theframes on each side of the diaphragm intermediate the rim and the centerregion in closelyspaced planar relation to the diaphragm, the frameshaving electrode support means intermediate said rim and center region,each electrode comprising a continuous wire having an insulating sheath,disposed in a flat spiral having spaced turns, the electrode supportmeans being spaced from the diaphragm a greater distance than thedistance from the diaphragm to a plane tangent to the insulating sheathsof the wires on their sides remote from the diaphragm, and bondingmaterial intermediate the sheaths and the support means.
 2. Anelectrostatic transducer according to claim 1 wherein the bondingmaterial is disposed in radial strips between center region and the rim.3. An electrostatic transducer according to claim 1 wherein the supportmeans for the electrodes comprise spaced radially disposed membersintermediate the central and rim portions of the frames and the bondingmaterial is intermediate the radially disposed members and adjacentportions of the insulating sheaths.
 4. An electrostatic transduceraccording to claim 1 wherein the electrode support means comprises a webhaving a multiplicity of small closely-spaced apertures and the bondingmaterial is disposed in radial strips on the web.
 5. An electrostatictransducer according to claim 1 wherein each frame has an intermediatering portion between the central region and the outer circular rim. 6.An electrostatic transducer according to claim 1 wherein a plurality ofradial supports extend between central disc and intermediate ring, andbetween the intermediate ring and the outer rim and the bonding materialis intermediate the radial supports and adjacent portions of theinsulating sheaths.
 7. An electrostatic transducer comprising a flexiblediaphragm having a conductive coating of high resistivity, a pair offrames each having a circular rim between which the margin of thediaphragm is clamped and a circular center region coplanar with the rimbetween which the center of the diaphragm is clamped, and conductiveelectrodes mounted on the frames on each side of the diaphragmintermediate the rim and the center region in closely-spaced planarrelation to the diaphragm, the frames having electrode support meansintermediate said rim and center region, each electrode comprising acontinuous wire disposed in a flat spiral having spaced turns, theelectrode support means being spaced from the diaphragm a greaterdistance than the distance from the diaphragm to a plane tangent to theinsulating sheaths of the wires on their sides remote from thediaphragm, and bonding material intermediate the sheaths and the supportmeans.